Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a tower and a rotor arranged on the tower. The rotor, which typically comprises a hub and a plurality of blades, is set into rotation under the influence of the wind on the blades. Said rotation is normally transmitted through a rotor shaft to a generator, either directly or through the use of a gearbox. This way, the generator can produce electricity which can be supplied to the electrical grid.
During operation of a wind turbine, the tower structure may undergo undesired vibrations, i.e. oscillatory or repeating displacements in any direction (fore-aft vibrations, side-to-side or lateral vibrations, longitudinal vibrations, torsional vibrations, etc.) of different amplitudes and frequencies (high or low, constant or varying). These vibrations may be caused by different factors, e.g. wind acting on the tower, blades passing along the tower and locally disturbing the wind flow, vibrations transmitted from the gearbox to the tower, rotor movements, nacelle imbalances, vibrations from the hub transmitted to the tower etc.
If a tower is subjected to vibrations during a prolonged period of time, fatigue damage may result. Fatigue damage may lead to a reduced life time of the wind turbine tower and/or its components. Furthermore, the danger exists that when vibrations cause resonance in the wind turbine tower, this can lead to a potentially dangerous increase of the vibrations. A further complicating factor is that the size of wind turbines (rotor, nacelle, tower, etc.) keeps increasing. Also, as towers become higher, the effect of vibrations becomes more critical.
A tuned mass damper, also known as a harmonic absorber, is a device mounted in structures to reduce the amplitude of structural vibrations. Their application can prevent discomfort, damage, or outright structural failure depending on the application. Tuned mass dampers are used to stabilize and protect against violent motion caused by harmonic vibration. A tuned mass damper aims at reducing the vibration of a system with a comparatively lightweight component so that the worst-case vibrations are less intense. Systems employing tuned mass dampers are tuned to either move the main mode away from a troubling excitation frequency, or to add damping to a resonance that is difficult or expensive to damp directly.
US2013280064 A1 discloses a wind turbine having a damper which may be a tuned mass damper. The damper is aimed at damping oscillations of the wind turbine, for example the periodic bending of the tower. The damper may be based on having a movable mass which can be a liquid (e.g. seawater) in a container.
A drawback of this type of wind turbine may be that the container and its content add an extra weight to the structure of the wind turbine. An increase in the weight of e.g. the nacelle may lead to a further increase in material and weight in the tower.
Another drawback may be that the container occupies a position and space which is not available for components that are essential to achieve the main purpose of the wind turbine: generation of electricity from the wind. The position and space occupied by the container may thus condition the position and space occupied by e.g. a main generator, a gearbox, a transformer, etc.
A further drawback of this type of wind turbine may be that the container may require its filling and/or emptying for e.g. maintenance reasons, tuning reasons (depending on the oscillations to be damped), etc. This filling and/or emptying may involve operations which may be relatively complex and expensive.
Still further drawbacks of this type of wind turbine may be that the container and possibly other related elements generally represent an extra cost in the sense of that they are not essential for the main purpose of the wind turbine (generation of electricity from the wind).
The present invention aims at resolving at least some of these problems.